Originally Posted by tpelle

Well, even though I've never done it, I can see how it could happen. You have the pump head firmly clamped on the valve stem, and there's little or no air pressure in the tube to stabilize things against the rim (Presta valves, of course, are better here as one can tighten the little lock ring down against the rim). You are trying to push 100+ PSI of air pressure in to the tube by working this flimsy telescoping contraption back and forth with the flagging strength of your little bicyclist-skinny arms. In doing so, the end of the pump clamped on to the rubber valve stem, which is protruding through a hole in a thin sheetmetal rim, is jerking the stem back and forth. All this is taking place while the bike is precariously leaning against a mailbox or tree or signpost, your knees hurt from kneeling on the loose gravel, grit, and broken glass shards on the shoulder of the road, and 18-wheelers are roaring past 18" from your posterior, and you're pissed because this is the second flat you've had in two days and you already used your spare tube so you were forced to use the patch kit and had trouble finding the puncture, so you're not really sure you're not going to have to be doing it all over again in a couple of minutes.

Yep, I could see how it could happen.